Process control systems, like those used in chemical, petroleum or other processes, typically include multiple field devices such as valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure, and flow rate sensors) disposed in various locations within a corresponding process plant to perform process control functions such as opening or closing valves, measuring process parameters, increasing or decreasing fluid flow, etc. Field devices typically communicate with one or more centralized or decentralized process controllers (which are in turn communicatively coupled to host or operator workstations) via analog, digital or combined analog/digital buses. Smart field devices such as field devices conforming to the well-known protocols such as FOUNDATION™ Fieldbus, Device-Net®, or HART®, may also perform control calculations, alarm functions, and other control functions commonly implemented within the process controller.
Some communication protocols used to transmit process control data to and from field devices are associated with a floating ground potential. In other words, the ground reference of devices using these protocols, or circuit ground, is allowed to float relative to the ground potential. To take one example, Foundation™ Fieldbus protocols require that Fieldbus communication links be isolated from ground. As one of the consequences of the floating ground requirement, power frequency noise coupled onto a Fieldbus network effectively modulates circuit ground of each participating device relative to ground. The actual amount of noise varies according to a particular installation, equipment, and certain environmental factors.
In general, several types of signaling on communication links having multiple wires or lines are known. A common-mode signal, for example, appears on both wires of a two-wire communication link. Accordingly, the common-mode voltage signal is the average of the two voltages, each voltage calculated relative to common ground. If the voltage on the first wire is V1, and the voltage on the second wire is V2, the common-mode voltage signal VCM is defines as:
                              V          CM                =                                                            V                1                            +                              V                2                                      2                    .                                    (                  Equation          ⁢                                          ⁢          1                )            
By contrast, a normal-mode signal VNM appears across the pair of wires in a two-wire configuration:VNM=V2−V  (Equation 2)It will be noted that if the ground potential changes, each of the voltages V1 and V2 changes by an equal amount relative the previous value of the ground potential, and the normal-mode voltage signal VNM accordingly remains unaffected:VNM=(V2+VΔ)−(V1+VΔ)=V2−V1  (Equation 3)
In addition to transmitting and receiving process control data using communication protocols such as Foundation Fieldbus, some field devices also communicate with auxiliary devices via separate communication links using various signaling techniques (e.g., normal-mode, common-mode, differential-mode, etc.) and various communication protocols. For example, a digital valve controller (DVC) may communicate with a controller via a Fieldbus communication link and with a remote travel sensor via an auxiliary independent wired communication link. Signaling on the auxiliary wired communication link may be more susceptible to noise than signaling on the Fieldbus communication link. In particular, common-mode noise may prevent the DVC from accurately receiving signals from the remote travel sensor, which may be in the millivolt range.